Studies are proposed to elucidate the reaction mechanism of the Na, K-ATPase. This ATPase is the enzymatic representation of the plasma membrane sodium pump, responsible for the transport of Na and K out of and into cells; consequently, the enzyme plays a crucial role in general cell physiology. Biochemical studies on three aspects of the reaction process are planned: (1) characterization of the nucleotide substrate sites, with particular emphasis on defining the enzymatic roles of the high-affinity and low-affinity substrate sites in the various reactions catalyzed by the ATPase; (2) description of the interaction between potassium ions and enzyme, with emphasis on those sites through which the phosphatase reaction is catalyzed and their relationship to sites through which nucleotide binding, tryptic digestion, and Na, K-ATPase activity are influenced: (3) chemical modification of the enzyme, particularly in terms of alterations in sensitivity to potassium ions. The major goal is a description of the molecular events underlying active transport.